A driver is typically used to drive an electrical signal onto a conductive path such as a trace, which is connected to a receiver. The signal requires a return path from the receiver back to the driver. More specifically, to generate a positive voltage signal in a theoretical sense, the driver generates the signal by removing positive charge from a supply node such as a VCC node, and driving the positive charge onto the conductive path to the receiver. Once the charge reaches the receiver, it returns to the VCC node of the driver via a return path having the least impedance (if there are multiple return paths to choose from). Unfortunately, the longer this path and the higher its impedance, the more likely the signal is to generate cross talk or electromagnetic interference (EMI). Often the driver and receiver are mounted on separate circuit boards that are connected by a flexible cable, or flex cable.
FIG. 1 is a block diagram illustrating a driver 10 located on a circuit board 1 and a receiver 20 located on a circuit board 2 and coupled by a flex cable 30 having a conductor 32 for conducting a signal IS. Typically, a high-speed electrical signal will switch between two voltages, here VCC and ground (GND). VCC is a voltage that is normally carried by power planes 12 and 22 on the boards 1 and 2, respectively, and GND is typically carried on respective GND planes of the boards 1 and 2. The flex cable 30 typically includes a ground trace or plane (not shown) that connects the GND planes on the boards 1 and 2. Here a “plane” is a conductor that covers virtually the entire area of one layer of the circuit board, and “trace” is a conductor that is much thinner than a plane, and thus covers only a small area of one layer of the circuit board. Thus, a circuit board layer may include many traces and no plane, or include only one plane but no traces.
Still referring to FIG. 1, the conductor 32 provides the signal IS path to the receiver 20. To complete the electrical circuit, the charge delivered to the receiver 20 must return to the VCC plane 12 via a return-signal IR path. Thus, a charge must flow from the power plane VCC 12, through the driver 10, the conductor 32, the receiver 20, and then a return path back to the power plane VCC 12. The return charge finds a path through the flex cable 30. Such a return path is typically long and has significant high impedance, such that a loop formed by the signal Is path through the conductor 32 and the return-signal IR path radiates significant amounts of EMI and causes significant amounts of cross talk.
FIG. 2 is a block diagram illustrating the components of FIG. 1 where the flex cable 30 includes a ground plane 36 that is connected to the ground planes of the boards 1 and 2, and a by-pass capacitor CB that couples the ground plane of the board 1 to the VCC plane 12 of the board 1. Although the ground plane 36 is an element of, and thus included within, the flex cable 30, it is shown outside of the flex cable 30 for illustrative purposes.
Because the ground plane 36 presents a low impedance path, the return charge typically follows a return path from the receiver 20 through the ground plane of the board 2 to the ground plane 36 of the flex cable, through the ground plane of the board 1 and the by-pass capacitor CB, to the VCC plane 12. The ground plane 36 can be selected by circuit designers for a return path because it typically has a lower high-frequency impedance than other possible return paths existing on the boards 1 and 2. Furthermore, using the ground plane provides a return loop that is typically smaller and has less impedance than other possible return paths, and thus generates less EMI and less cross talk. However, the by-pass capacitor CB typically has a non-zero impedance even at high frequency, and thus may present an impedance mismatch between the ground plane of the board 1 and the VCC plane 12. Unfortunately, this mismatch may still cause the signal loop to generate significant levels of EMI and cross talk.
Although a signal IS generated from a positive supply VCC is discussed, the claimed invention is applicable to a signal that is generated from a negative supply voltage as well.